Ultrafast nonlinear optics of bulk and two-dimensional materials for infrared applications
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This thesis presents the results of an experimental study into the nonlinear optical properties of novel nonlinear materials at infrared regions of the electromagnetic spectrum for the realisation of nonlinear optical devices in the near- and mid-infrared. Because of its exceptional nonlinear optical properties and its promise of implementation in a range of mid-infrared applications graphene had a prominent place in this research. Extensive investigations in the nonlinear optical properties of single and multilayer chemical vapour deposition (CVD) graphene are presented. This study revealed that graphene presents a nonlinear phase shift due to a negative, irradiance-dependent nonlinear refraction. The high peak powers available enabled the study of both saturable absorption (SA) and two-photon absorption (2PA), identifying the irradiance limits at which the contribution of two-photon absorption exceeded that of saturable absorption. Moreover, the nonlinear optical properties of graphene-polyvinyl alcohol (G-PVA) composite films were studied. The results indicate the thermal damage of the host polymer due to graphene heating and temperature transfer. Studies in the third order nonlinear optical properties of chalcogenide glasses with the perspective of integration with graphene for the development of mid-infrared devices and applications are also performed. Of all the glasses investigated, gallium lanthanum sulphide (GLS) was found to have the most interesting nonlinear optical properties. Its optical Kerr nonlinearity was found to be approximately 35 times higher than silica and the upper limit of its two-photon absorption coefficient was the lowest of all the chalcogenide glasses analysed, implying that GLS would be an excellent candidate for ultrafast all-optical switching. Subsequently GLS was chosen as the host material for optical waveguide and device fabrication via ultrafast laser inscription (ULI). Near- and mid-infrared waveguides were successfully fabricated; fundamental features such as, refractive index profiles and material dispersion were investigated. The Zero Dispersion Wavelength (ZDW) of GLS was experimentally measured for the first time; the ZDW was determined to be between 3.66-3.71 μm for the waveguides and about 3.61 μm for the bulk. Single mode directional couplers at 1550 nm were also developed and their ultrafast all-optical switching properties were investigated, leading to the assessment of the nonlinear refractive index n2 of the ULI modified area. Furthermore, waveguides in Er3+ doped GLS were successfully fabricated and the infrared transitions at 1550 and 2750 nm were detected opening the potential for GLS waveguide lasers.